1. Field of the Invention
The present invention relates to semiconductor laser modules, in particular, to a semiconductor laser module configured by forming a resonator configuration with an optical amplifier and an optical waveguide, a semiconductor laser, and a method of assembling the same.
2. Description of the Related Art
Recently, optical communication modules such as optical semiconductor laser module and light receiving module are becoming more and more miniaturized with higher speed and larger capacity of the optical communication system. Accompanied therewith, optical elements such as semiconductor laser, optical filter, optical lens etc. are also becoming miniaturized.
Under such conditions, a method (hereinafter referred to as “active alignment”) of aligning the optical axis to an optimum position while discharging or transmitting light from the optical element is generally widely used when mounting the optical element. When performing active alignment, a method of fixing the optical element on a ceramic plate called carrier, and aligning the optical axis while moving the carrier so that three axes position of X, Y, Z and three angles of θX, θY, θZ become optimum is adopted.
In the case of an external resonator laser module in which the external resonator configuration is formed with the optical amplifier and the optical waveguide to serve as the optical element, high precision active alignment is necessary to obtain a desired optical output while suppressing coupling loss and to obtain a stable laser oscillation while suppressing reflection. Furthermore, since the optical elements are enclosed in an air tight package for use in the usual module, the optical system is required to be fixed in a high precision active aligned state in the external resonator laser described above.
Fixation with adhesive that enables fixation at higher precision than fixation by YAG laser is more frequently used to suppress increase in coupling loss caused by axial shift and angular shift in time of fixation. A combination of the optical amplifier and the optical waveguide has been used in the above description, but the combination is not limited thereto, and it is also applicable when active aligning the coupling between the laser element and the waveguide element.
FIG. 8 shows a semiconductor laser module 100 of the external resonator configuration. In this example, a resonator configuration is configured by an optical amplifier 51 and an optical waveguide 53 to form the semiconductor laser 50. In this case, the optical system is required to be fixed in a high precision active aligned state. The fixation with adhesive, with which the shift in optical axis in time of fixation is small, is frequently used. That is, in the semiconductor laser module 100 shown in FIG. 8, element carriers 52, 54 holding the optical amplifier 51 and the optical waveguide 53, respectively, are aligned to the optimum position, and the element carriers 52, 54 are fixed with adhesive in an abutted state. FIG. 9 shows a state in which the element carriers are fixed with adhesive in the abutted state. Reference numeral 55 indicates a contacting part of the element carriers 52, 54.
As shown in FIG. 8, the element carriers 52, 54 are arranged on a Peltier cooler 56 having a cooling function, so that the temperature of the optical amplifier 51 and the optical waveguide 53 is temperature controlled at a level of 1/100 [° C.]. The laser light output from the optical amplifier 51 is output towards an external optical fiber 57 via an optical system 58. The optical system 58 includes a first micro-lens 58A fixed and arranged on the element carrier 52 that holds the optical amplifier 51, and a second micro-lens 58B arranged inside a connecting part 59 for connecting the optical fiber 57.
In FIG. 8, reference numeral 60 indicates a case main body. A sealing lid is arranged on the upper surface of the case main body 60. Reference numeral 60a is a pass-through hole formed in correspondence to the connecting part 59 of the case main body 60, and defines a space for laser passage between the first and second micro-lenses 58A, 58B.
The first micro-lens 58A collimates the laser light discharged from the optical amplifier 51. The collimated light is collected at the second micro-lens 58B, and entered to the optical fiber 57. The Peltier cooler 56 temperature controls the entire module, thereby ensuring stable laser oscillation.
A conventionally known optical coupling device (patent documents 1 and 2) similar in technical matter with the present invention will now be described in addition to the example of FIG. 8 described above.
In the example disclosed in Japanese Laid-Open Patent Publication No. 2003-57467 (patent document 1), the semiconductor laser and the optical fiber are optically coupled using the optical waveguide, where a portion holding the fiber is integrally molded with a rising part of an L-shaped holding member (hard member), and a bottom part of the L-shaped holding member is formed with a cut-out for the optical waveguide from the end part towards the central part. The relevant example has features in that the optical waveguide is arranged and securely attached to the cut-out, the semiconductor laser is attached to the end part side of the optical waveguide, and a core for optically coupling an active layer of the semiconductor laser and the optical fiber is arranged on the optical waveguide, thereby forming the optical waveguide module.
In the example disclosed in Japanese Laid-Open Patent Publication No. 5-210026 (patent document 2), a block for the optical waveguide (optical waveguide block) is attached to the end of each optical fiber when coupling the optical fibers by way of the optical waveguide, and the optical waveguide block is partially welded and integrated with the YAG laser by way of the optical waveguide, thereby forming the coupling configuration of the optical fiber.
In the example shown in FIG. 8 described above, the element carrier 52 and the element carrier 54 are aligned to the optimum position and fixed with adhesive in an abutted state. Thus, if the abutting force is weak, the adhering surfaces of the two element carriers do not closely contact and optimum alignment cannot be performed, and furthermore, variation in thickness of the adhesive occurs thereby causing shift in optical axis after fixation. The abutting force thus is required to be strong.
If the area of the abutting surfaces of the element carriers is narrow, however, stress in the bending direction with one point or one side of the abutting surface as the supporting point is generated when the abutting surfaces are even slightly non-parallel in abutment, and shift further occurs in the bending direction the stronger the abutting force, whereby optimum alignment cannot be performed.
For instance, as shown in FIG. 10, if the element carriers 52, 54 having a small size of about 1-2 [mm] of about the same as the optical amplifier 51 and the optical waveguide 53, which are optical elements, are used, the element carriers tilt when subjected to the stress in the bending direction in time of abutment, whereby the coupling loss increases and the desired optical output cannot be obtained, or the reflection at the end face increases and a stable oscillation state cannot be obtained. The symbol a indicates the tilt angle in this case. When an element carrier including a large contacting area is used to alleviate the stress in the bending direction, the size of the element carrier increases and the package size for enclosing the same increases, and thus is not suited for miniaturizing the module.
In the example shown in patent document 1, the entire side surface of the optical waveguide for holding the semiconductor laser is further held by the L-shaped holding member (hard member), and thus the L-shaped holding member is largely arranged on the side surface of the optical waveguide, whereby the durability of the device strengthens but the entire device enlarges and general versatility tends to lack.
Furthermore, in the example shown in patent document 2, the optical waveguide block with a relatively large contacting surface is arranged on both optical fibers by way of the optical waveguide, and the optical waveguide block and the optical waveguide are partially welded and integrated with YAG laser when coupling the optical fibers by way of the optical waveguide, whereby the entire device enlarges and general versatility tends to lacks, and furthermore, problem arises in terms of durability of the welded portion.